You may have heard that you work different muscles in an elbow flexion exercise depending on which
position your hand is in. Palm up works the biceps brachii, palm neutral works the brachioradialis, and
palm down works the brachialis. But why is this a thing on an anatomical level? In today's video
we'll look at the anatomy of the elbow so that we can understand how such similar exercises
can work entirely different muscles.
Hello and welcome.
If you're new here my name is Patrick and this channel is all about anatomy and physiology.
We're going to build on the big bones of the arm:
there's the humerus or upper arm bone, the ulna, which is the medial arm bone running from the hard tip of your elbow to the pinky side of the wrist.
Then there's also the radius, which runs from the
lateral elbow to the thumb side of the wrist. The humerus looks like a smooth ball and socket at the
shoulder but it's got a bunch of tiny bumps and grooves that allow muscles to stay in place while
the shoulder joint is moving around. More obvious are the bumps and grooves at the distal end which
is where the humerus meets the radius and ulna to make the elbow joint. Each of these flared tips
are the epicondyles. This is the medial epicondyle which is where all the forearm flexors originate
and this is the lateral epicondyle where the forearm extensors originate. This here is called
the trochlea and it creates a smooth surface for the ulna's trochlear notch to glide in.
This articulation is how the elbow flexes and extends as a traditional hinge joint. It can only
move in one direction, flexion and extension, that's it. The piece of anatomy that answers the elbow
curl question is the radius. At the elbow joint we find the proximal end of the radius: the concave
radial head. It's shaped kind of like a golf tee and it pivots on the smooth rounded surface of
the anterior humerus called the capitulum making it one of the few pivot joints in the body. You
can palpate it on yourself by starting at the lateral epicondyle and going about an inch down
the forearm. Flip your forearm back and forth a couple times and you'll feel the radial head poke
out. That radial head is attached to the ulna by a circular piece of connective tissue called the
annular ligament and the fact that the radius can pivot is what allows us to flip our hands
up and down or supinate and pronate. While it seems like your forearm is flipping entirely around, your
ulna isn't moving. Your radius is moving around your ulna,. As a result we have to pay attention
to which elbow muscles tug on which bones because if they connect the upper arm to the radius then
hand position will influence how they contract in elbow flexion but if the muscle attaches to the
ulna it won't be affected by hand position so let's get through them. The deepest of the elbow
flexors is the brachialis. It's a squatty muscle that originates on the distal half of the anterior
aspect of the humerus -- basically from the sleeve down -- and that big wide muscle converges into a
thick tendon that inserts on the anterior ulna on a bump called the tuberosity of the ulna.
(a little bit of the coronoid process too). Because it connects the humerus to the ulna, the brachialis is
always contracting during elbow flexion regardless of hand position. Superficial to that muscle is
the biceps brachii. Bi-ceps means two-heads, each of which originates up on the shoulder. The long head
originates on a little bump just a smidge superior to the glenoid fossa of the shoulder joint. It has
a long tendon that wraps around the anterior head of the humerus. Meanwhile the short head of the
biceps originates on that weird-looking projection called the coracoid process of the scapula and
since the bicep crosses the shoulder joint it's responsible for a little bit of shoulder flexion
Those two heads fuse into this big muscle belly and insert on the radial tuberosity
just distal to the head and neck of the radius. But there's also a thin sheet of connective
tissue that stems from that insertion called the bicipital aponeurosis. This thing separates some
of the deep muscles from the superficial ones and lies on top of the deeper forearm muscles. And that
insertion ends up being important for two reasons. Number one, it's on the radius so unlike the
brachialis which inserts on the ulna, the biceps' mechanics will be influenced by forearm position.
And #2, that connective tissue can get in the way of bicep function, but we'll come back to
that. Finally, we've got the brachioradialis which originates on the lateral aspect of the distal
humerus on the ridge just superior to the lateral condyle (this general area here). The bulk of the
muscle belly is found within the proximal half of the forearm at which point it narrows off into a
thick tendon and inserts around the end of the radius. Just like the biceps, the brachioradialis
inserts on the radius so it is affected by forearm position and actually can supinate
or pronate the forearm itself. I mentioned origins and insertions because they influence a few things
like how much torque the muscle can produce. I want you to imagine your elbow joint like this
barbell set up here. The barbell itself represents your forearm. The place where the barbell meets
the wall represents the elbow joint. And that weight on the end represents a weight in hand.
Now you intuitively know that it's going to be easier to lift that weight the farther away we get
from the axis. In anatomy that distance between the axis and the force applied is called the moment
arm. For the weight, that's the external moment arm. For the effort that our muscles provide
that's the internal moment arm. The brachialis and the biceps brachii have really short moment arms.
They insert pretty close to the axis. Meanwhile the brachioradialis inserts way farther down closer
to the tip of the radius so it has a long internal moment arm. That means that the brachioradialis can
put forth less force than the biceps brachii and still lift the same external weight.
The second reason we need those O's and I's is because different muscles are advantaged or
disadvantaged depending on their position. What do I mean by advantage? In reality, all of these
muscles will help each other in each exercise at least a little bit. But different positions
can tweak how much they contribute to the overall lift. This is the concept that finally explains how
forearm position impacts muscle activation during a bicep curl. When the forearm is fully pronated
the radius is twisted all the way over the ulna. This causes that sheet of tissue (the bicipital
aponeurosis) to nudge up against the biceps main insertion on the radial tuberosity. This puts the
bicep brachii at a disadvantage. Since the biceps is handicapped though, the brachioradialis has
to take more of the load but its fibers still aren't running totally parallel so it's not in
an optimal position either. With those two muscles generating less than ideal torque,
the bulk of the load is taken up by the only muscle left: the brachialis. This is why fitness
coaches will tell you that the prone curl works the brachialis. But remember, since the brachialis
attaches to the ulna it's unaffected by forearm pronation or supination. It's always contracting
during elbow flexion. Now when the forearm is in a semi-pronated position, the biceps insertion
is still at a bit of a disadvantage, but luckily now we're in a position where the fibers of the
brachioradialis are parallel to its origin. So in the hammer curl, the brachioradialis can contribute
to flexion more than in the prone position. Finally, when the forearm is fully supinated,
now the biceps insertion has a clear pull on the radius and can give its all to elbow flexion. Plus
the bicep is a supinating muscle so it's pulling in exactly the path that it's built for. Again,
the other elbow muscles are still activated but contracting into supinated forearm elbow flexion
is the optimal position for biceps contraction. So the big takeaway from this video is that, yes, these
different forearm positions can work different muscles. Not because we can turn on or off muscles
but because of how they attach to the forearm. If you want to learn more about arm muscles check out
this video I did about palmaris longus but go easy on me. It was my very first Corporis video
ever from back in 2018. Otherwise, thank you to my patrons on Patreon. You can get early access to
scripts and videos for as little as two dollars a month. Have fun, be good. Thanks for watching.